The deaf and the hearing-impaired who cannot hear well enough to use the telephone, use communication terminals to converse via written messages over telephone lines. Such terminals are referred to as telecommunication devices for the deaf, or TDDs, and typically include a typewriter-like alphanumeric keyboard and a display connected to the telephone through a modem (modulator/demodulator).
When characters are typed by a first party on the keyboard of a first terminal, tones corresponding to the characters are transmitted via the modem through the telephone line to a second terminal. At the second terminal, the tones are received by a second modem and converted back to characters to be read by a second party. In this manner, a conversation proceeds by the taking of turns by each of the users in the typing back and forth of TDDs. TDD terminals may also transmit certain control or data characters which henceforth will be generally referred to as text even though they may not result in a visible text display.
The rules for converting text into tones generally follows the Baudot/Weitbrecht protocol in which the transmitted tones include a mark tone of 1,400 Hz. and a space tone of 1,800 Hz. Sequences of marks and spaces provide five bit binary numbers representing a limited set of letters of the alphabet and certain other characters.
The Baudot/Weitbrecht protocol is a simplex protocol meaning that transmissions in both directions along the telephone line cannot be sustained without interference. When information is sought to be sent from both terminals at the same time, the net result is that the transmission will be garbled and the text displayed erroneous.
The transmission of text under the Baudot/Weitbrecht protocol is relatively slow. Characters are transmitted by the modem over the telephone lines at approximately six characters per second. Significantly, many adept typists among TDD communicators are able to type at rates in excess of six characters per second.
The net result is that each TDD may spend a large portion of its time actively transmitting and receiving, making ever present the possibility that two TDDs will be transmitting at the same time. In order to avoid interference in a Baudot/Weitbrecht system, it is therefore common in the United States to type the letters "ga" as an abbreviation for "go ahead" at the end of the text string to indicate to the other user that it is his or her turn to type.
It is often necessary or desirable for a TDD user to call a telephone number that is not equipped with a TDD. These calls may be either emergency or nonemergency, e.g. telephone calls to police, employers, doctors, repair and maintenance workers, relatives and loved ones, etc. In order to make possible telephone calls between a TDD user and a non TDD equipped telephone number, TDD relay centers have been established in many locations. A TDD relay center receives telephone calls from TDD users who request, via TDD, for a relay operator to place a call with a second party for them. The relay operator then calls the second party and tells the second party what the TDD caller is typing. When the second party talks, the operator then types back to the TDD caller. Conversely, a call through the relay center may originate from a non TDD user wishing to communicate with a TDD user. The call then proceeds in the same manner.
Deaf people who can speak may prefer to speak directly to the other party and have the relay operator only transcribe the response. Similarly, a speech impaired caller may prefer to listen for themselves and to have the relay operator simply recite the written half of the conversation. In the previously cited U.S. Pat. No. 5,081,673, a voice carryover bridge is disclosed which may allow a voice or TDD signal in one direction to pass through the relay without intervention by the relay operator. This voice carryover increases the privacy of the callers and reduces the effort required of the relay operator who need not distinguish between two voices and must transcribe or recite only half as much of the conversation. Further, the voice carryover permits communication of information between the parties normally lost from displayed text, such as: identity of the caller, tone, inflection, etc.
Families who have both members who are deaf and that can hear, typically have a TDD and a telephone sitting next to each other both for the convenience of each member of the household and to insure that incoming calls may be correctly answered with the appropriate type of equipment. Clearly this is inefficient in terms of both use of space and duplication of hardware. Institutions and public facilities that wish to receive and transmit both TDD text and voice face a similar issue of duplication.